1. Field of the Invention
The present invention relates to information recording mediums such as discs, cards and tapes utilizing light or magnetization for recording/reproducing an information recording signal on/from the mediums, particularly, related to information recording mediums suitable for recording/reproducing type recording mediums utilizing magneto-optical or phase change phenomenon, support members (supporters) used for forming a recording layer of the medium thereon, manufacturing methods and apparatuses for manufacturing the supporters, and stampers for producing the mediums.
2. Description of the Related Arts
In the field of optical discs in the prior arts, there are recording/reproducing type discs such as a MD (mini disc) capable of recording music information and a DVD (digital versatile disc)-RAM capable of recording data. Further, recently, in even the field of floppy discs, which were only utilized in magnetic recording in the prior art, there has been developed an optical floppy disc. As a typical example, there has been developed an optical floppy referred to as a supper-disc employing an optical tracking. Furthermore, in the field of the cards in the prior art, the magnetic card was a main stream of the cards. Recently, however, an optical card is beginning to be seen in the market. Further, in even the field of the magnetic tapes there have been developed not only magnetic tapes but also optical tapes.
As mentioned above, the optical techniques are not a monopoly on the disc type information recording mediums but are applied to all kinds of recording mediums having various shapes. In any cases, the developments of the recording mediums are forwarded to realize a high recording density and large capacity recording.
In order to realize a high density recording of the optical disc, there are utilized optical techniques such as a super-resolution reproducing techniques and a land/groove recording technique. However, many problems are pointed out in these techniques, resulting in constraints on realizing a further high recording density.
Next, a description is given of an example of the super-resolution reproducing, i.e., a DWDD (Domain Wall Displacement Detection) type information recording medium and its problems.
Specifically, the DWDD type information recording medium refers to magneto-optical information recording medium such as a magneto-optical disc and a magneto-optical cards, wherein on a supporter member (referred to as a supporter hereinafter) a recording layer and a protection layer are laminated in this order. The recording layer comprises a displacement layer having a small wall coercivity, a switching layer having a relatively lower Curie temperature Ts and a memory layer having a relatively large wall coercivity, resulting an exchange-coupling triple-layered magnetic film known as a super-resolution layer.
Upon reproducing, when the recording layer is heated by being locally irradiated with a laser beam thereon, a temperature gradient is developed in the recording layer. This temperature gradient causes force to drive the domain wall of the displacement layer against the wall coercivity thereof in the higher temperature direction because the wall energy decreases as the temperature increases. When the domain wall of the displacement layer is located in a region where the temperature is lower than the temperature Ts, the domain wall can not be displaced due to the large frictional force from the memory layer through the exchange coupling acting between the displacement layer and the memory layer through the switching layer. However, along with the laser beam movement, when the domain wall is transferred into a region where the temperature is higher than the temperature Ts, the exchange coupling between the displacement layer and the memory layer through the switching layer is disappeared because the magnetization of the switching layer is vanished, and as a result, the domain wall in the displacement layer is solely displaced in the higher temperature direction. The domain wall displacement in the displacement layer is developed each time when a domain wall which is formed at an interval corresponding to an information signal, reaches the isothermal line of the Ts.
In other words, as the recording medium is scanned at a constant speed by using the laser beam, this domain wall displacement mentioned above occurs at a time interval corresponding to a spatial interval of the recorded domain wall. Thus, the information recording signal is reproduced by detecting magnetic reversals associated with the domain wall displacement by using a conventional magneto-optical system irrespective of the resolution of an optical readout system used in the apparatus.
FIG. 5 is a schematic sectional view of a supporter used in an information recording medium in the prior art, and FIG. 6 is a schematic sectional view of an information recording medium employing the supporter shown in FIG. 5 in the prior art.
FIG. 5 shows a supporter 7 used for a DWDD type information recording medium, and FIG. 6 shows the DWDD type information recording medium employing the DWDD type supporter 7 shown in FIG. 5 on which a recording layer 5 and a protection layer 6 are laminated in this order.
Referring to FIG. 5, on the supporter 7 there are formed optical tracking grooves (referred to as grooves hereinafter) 3 having a flat bottom, as a minute track pattern. For example, in a case where the supporter 7 is used for a disc-type information recording medium such as an optical disc, the grooves 3 are formed circularly or spirally. And, between adjacent grooves 3, there is formed a flat hill referred to as a land 2. As shown in FIG. 6, on a plane of the supporter 7 formed with the land 2 and the grooves 3, there are formed a recording layer 5 and a protection layer 6, resulting in an information recording medium A. The recording layer 5 is made of a super-resolution magneto-optical layer, for instance, a triple-layered film composed of a displacement layer made of GdFeCr, a switching layer of TbFeCr and a memory layer of TbFeCoCr.
In order to derive the excellent characteristics from this recording layer to a maximum, the recording layer 5 may be interposed between subsidiary dielectric layers. Here, the recording layer including the subsidiary layers is designated as the recording layer 5. The protection layer 6 is provided for protecting the recording layer 5, and is made of a thick resin layer made of an ultraviolet curing resin or a heat curing resin.
As to a principle of the DWDD, it is explained in detail in 1998 National Convention Record, the Institute of Electrical Engineers of Japan, S. 10-7 (page, S. 10-25 to 28). Thus, a detailed explanation thereof is omitted here for simplicity. However, it should be noted that an information signal is recorded on either a group of the lands 2 or a group of the grooves 3. At that time, either the group of the lands 2 or the group of the grooves 3 is made to be a non-magnetic area and the other is made to be a magnetic area. One of the most significant points to realize the DWDD reproducing is that a designed super-resolution phenomenon is never developed as far as both the group of the lands 2 and the group of the grooves 3 are made to be the magnetic area. In other words, the DWDD is not developed in the information recording medium A shown in FIG. 6 as it is. Thus, it is necessary to convert a part of the magnetic area into a non-magnetic area corresponding to non-recording tracks by continuously heating the part of the magnetic area with a laser beam from a recording/reproducing pickup at a higher power than that used in the recording.
FIG. 7 is a schematic sectional view showing a state where the lands of the DWDD type information recording medium shown in FIG. 6 are annealed, wherein the grooves 3 are made to be recorded tracks and the lands 2 are made to be non-magnetic tracks.
Specifically, before recording an information signal on the grooves 3, the magnetic area of the lands 2 is converted into a non-magnetic area by continuously scanning the lands 2 with the laser beam at a higher power L than that used in the recording. This process is referred to as a laser annealing process hereinafter. In FIG. 7, a hatched portion represents an annealed portion. Thus, the magnetic coupling between the lands 2 and the grooves 3 are disconnected, resulting in a DWDD type information recording medium 40. After the laser annealing process, it is possible to record/reproduce an information signal on/from the grooves 3, resulting in a development of the super-resolution phenomenon in the reproducing. This means that it is possible to read out a recorded information signal having a signal length smaller than that of the optically readable minimum signal length which is calculated by using both a wavelength of a laser beam and a numerical aperture of an objective lens used in the reproducing process.
The DWDD technique mentioned above is an excellent super-resolution recording/reproducing technique in principle, however, there is no alternative but the laser annealing to disconnect the magnetic coupling between the lands 2 and the grooves 3. This fact causes a problem of poor mass-productivity of the information recording medium. For instance, it takes a long time of 30 to 90 minutes for the laser annealing process because every track has to be scanned with the laser beam. Further, there is a theoretical problem that it is impossible to record an information signal on both the lands 2 and the grooves 3 because it requires an alternate construction of the magnetic area and the non-magnetic area. Thus, such a land/groove recording as utilized in a DVD-RAM can not be realized, resulting in a limitation to realize a large capacity recording.